Disruption of rcnB modulates colistin susceptibility in Acinetobacter baumannii AB5075.

Acinetobacter baumannii AB5075 is a clinically relevant multidrug-resistant (MDR) isolate that poses a major therapeutic challenge. Although colistin has been reinstated as a last-resort antibiotic against MDR Gram-negative infections, the rapid emergence of colistin resistance threatens its clinical utility. Here, we employed a CRISPR-Cas9-based genome editing system to generate an A. baumannii AB5075 ΔrcnB mutant and uncovered a previously underappreciated role of rcnB in modulating colistin susceptibility. Loss of rcnB markedly potentiated colistin-mediated killing through multiple associated changes, including compromised membrane integrity, impaired oxidative stress defenses, and reduced efflux pump activity. Transcriptomic profiling further revealed that rcnB deletion reshaped global stress-response networks, including suppression of fatty acid biosynthesis and reactive oxygen species (ROS)-detoxifying pathways, alongside altered metal ion and sulfur metabolism during colistin exposure. Collectively, our findings suggest that rcnB may contribute to colistin susceptibility of colistin resistance and provide mechanistic insights that may inform the development of targeted strategies to enhance colistin efficacy against MDR A. baumannii.